This invention relates to curable ethylene copolymer compositions which are especially adapted for continuous pressureless curing processes.
Elastomeric compositions require a vulcanization, i.e. curing, step in order to develop the crosslinked network structure which confers rubbery properties to such compositions. Typically, the curing processes are based on compression molding, transfer molding, or injection molding techniques wherein a fully compounded elastomer composition is introduced into a mold which is then heated under pressure, thereby producing a cured product. Pressureless curing processes are also known and these methods generally involve extrusion of fully compounded polymer compositions at temperatures below that at which vulcanization occurs, followed by a second step wherein the extrudate is cured by exposure to conditions which promote formation of covalent crosslinks. For example, the extrudate might be passed through a hot liquid curing bath, a fluidized bed, or a continuous hot air tunnel. Such continuous processes, which are generally more economical than molding processes, are particularly suited for preparation of coated wire, sheet, and elastomeric profiles.
Pressureless cure processes are not suitable for all elastomer compositions, however. For example, many elastomeric ethylene copolymers having polar comonomer units display unacceptable sponging and porosity when cured by these techniques. Such effects are possibly due to inherently low polymer and compound viscosity, generation of volatiles during the curing process, and/or inadequate crosslinking during the early stages of the curing process.
Ethylene copolymers containing polar comonomers have unusually good heat and oil resistance. These properties are particularly desirable in compositions used in automotive applications, such as in vacuum hoses and radiator hoses wherein oil resistance is a critical property and in automotive wiring where heat resistance is important. Because continuous, pressureless cure processes are the most desirable manufacturing methods for producing extruded seals and coating wire, there is thus a need in the art for a composition which can withstand such processing conditions.